Resonators are commonly used in electronics for providing a signal with accurate and stable frequency. The resonators are mostly made using quartz crystals, which have a good accuracy and temperature stability of frequency. However, the production process for producing crystal resonators is different from the process of producing most other electrical circuits, which are mainly produced of silicon. Therefore, the quartz crystal resonators are usually separate components, whereby separate phases are required in the production process of electronic devices. The quartz crystal components also tend to be large in size.
It would be desirable to provide MEMS resonators made of silicon in order to facilitate integration with other silicon based components. However, silicon based resonators have a disadvantage of high temperature drift of the resonance frequency. The drift is due to the temperature dependence of the Young modulus of silicon, which causes a temperature coefficient of approx. −30 ppm/C. This causes the resonance frequency to fluctuate due to changes in ambient temperature.
It is possible to compensate the temperature dependence with a temperature sensor and related electronic control circuitry, but it has not been possible to provide a resonator with sufficiently low temperature drift with low cost technology which would be suitable for mass production applications. Also, the use of a temperature compensation circuit increases the consumption of energy, which is a significant disadvantage especially in battery operated devices. Further, the compensation circuit tends to increase electric noise in the resonator circuit. It is also possible to stabilize the temperature of the resonator with temperature isolation and controlled warming/cooling of the resonator. However, this solution also increases the energy consumption of the device, and makes the device complicated to produce. The temperature compensation circuits are also slow in controlling, and cannot therefore compensate fast or large changes in ambient temperature sufficiently well.
It has also been suggested to use composite structures in resonators where there are layers with opposite temperature coefficients. Document U.S. Pat. No. 4,719,383 [1] discloses a shear wave resonator structure wherein a resonating beam has a piezoelectric layer and a p+ doped silicon layer. While the piezoelectric layer has a negative temperature coefficient, a heavily p+ doped silicon layer has a positive temperature coefficient. The thicknesses of the piezoelectric and doped silicon layers are made such that the total temperature coefficient of the resonator is near to zero.
There are certain disadvantages related with resonators of such composite structure as well. Firstly, the p+ doping of document [1] is made by diffusion via the material surface. Diffusion is typically a slow process, and therefore the doped layer cannot be very thick. Increasing the thickness of the silicon layer would also cause the coupling of the actuation to be worse. As a result, since the resonance frequency is a function of the total thickness of the resonator structure it is only possible to provide resonators with high frequencies. The patent document mentions suitable frequencies above 300 MHz. However, there are numerous applications where lower resonance frequencies are required, for example in the range of 10-40 MHz. The solution of document [1] is not feasible for such lower resonance frequencies.
Another problem relating to the composite structure of document [1] relates to the accuracy of the resonance frequency. In a thickness oriented shear wave resonator the resonance frequency is determined by the thickness of the resonator structure, and therefore an accurate resonance frequency requires achieving an accurate thickness of the beam structure. However, it appears very difficult to achieve sufficient accuracy of the thickness, and therefore it is difficult to achieve the required accuracy of resonance frequency. In mass production, the deviation of resonance frequencies of such resonators tend to be high, and thus the yield of resonators which fulfil the required specifications tends to become low.
A further problem which relates to the prior art MEMS resonators is the fact that the small-size resonator beam has a small oscillating mass, and therefore the resonator is able to store only a small amount of oscillation energy. This in turn causes a low signal-to-noise ratio of the resonator and thus instability of the output signal frequency.
A still further problem is related to actuation of the beam resonator where the beam oscillates in a thickness oriented shear wave mode. When piezoelectric actuation is used the c-axis of the piezoelectric layer must be inclined in order to provide actuation of the correct direction for the shear wave by using an electrical actuation field which is perpendicular to the plane of the dielectric layer. However, it requires special production technology to achieve a piezoelectric layer with inclined c-axis, and such a special technology is not commonly used in the production of microelectronics.